DESCRIPTION Programmed cell death (PCD) has been shown to play a crucial role in all models of metazoan development, from C. elegans to the mouse. Many cells that appear during development simply do not exist in the adult animal and it has been shown directly (in the case of C. elegans) or indirectly (in other developmental models and humans) that the extra cells die during progress to the adult form. The origin of PCD is obscure, but it is apparent that at some stage during the evolution of multicellular organisms the requirement to delete supernumerary cells became vital in establishing the optimal pattern if a functional adult. The ability to target the destruction of specific cells was an important development in the progress towards complex higher animals, and essentially the same system is used to achieve selectivity of the immune response and maintain cell number in adults. The system had even been co-opted to allow selective killing of virally-infected and transformed cells. However, a cell that is able to undergo PCD is inherently vulnerable. If the program is inappropriately activated the cell and its function are lost, and it is clear that inappropriate cell death contributes to the pathology of several human diseases, in particular neurodegenerative diseases. It is not clear why cell death leading to disease is most frequently associated with neuronal degenerative diseases, but there appear to be two possibilities. One suggests that neuronal cells contain a program that is more readily triggered than other cells, and the other suggests that cell death may occur on an organism-wide scale, but that degeneration of the neuronal system is more serious than degeneration of other organs. to distinguish these we must understand basic mechanisms of PCD, with special reference to neuronal cells. Proteolytic enzymes are key mediators of PCD, and there is now good evidence that members of a recently discovered family of proteases - the caspases - are responsible for causing execution of many of the morphologic changes associated with apoptosis. In certain types of PCD caspases can even initiate a pathway to apoptosis. The applicants hypothesize that those neurodegenerative diseases showing clear signs of apoptosis are caused by inappropriate activation of caspases, and that resolution of symptoms of the disease should therefore be achieved by controlling the activity of the culprits. The research covered in this proposal will determine the role of caspases in the initiation and execution of PCD in the neuronal cell line NT2. It will determine the requirements for activity of caspases that execute apoptosis, and then test the hypothesis that the disease-causing effect Alzheimer's Disease is due to apoptosis inappropriately triggered by lysosomal disruption.